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Severe weather is not a new thing. We only have to look back through New Zealand’s history to find many examples of appalling weather that have been deadly and wrecked communities and livelihoods.
In the past decade or so it has become steadily easier to detect the hand of global warming stirring things up somewhere in the background, whose influence is particularly evident from how extreme some storms are becoming and perhaps more so how frequently they are now occurring. However, quantifying that climate-change stimulus has been nigh-on impossible.
Until now. In what will be a great leap forward for preparedness, planning and public policy-making, New Zealand scientists are able to pinpoint with a good level of accuracy how much worse the Earth’s rapidly warming atmosphere has made any given severe weather event.
These climate-change attribution studies have been under way overseas for a few years, with researchers investigating some of the larger-scale northern hemisphere weather events, such as the scorching European heatwaves in mid-2019 and the heavy rains and flooding there in July this year.
The work of these scientists is highlighted on a website, World Weather Attribution, along with some of their other findings of how the climate influenced extreme rainfall in Texas from Tropical Storm Imelda in September 2019, and how it enhanced the August 2017 Bangladesh floods and torrential rain in southwestern Japan in July 2018.
New Zealand’s extreme weather events have not really been on this group’s radar because of their often limited geographic extent here and the relatively small number of people involved, perhaps hundreds or thousands compared with millions in Europe, Asia and North America.
But thanks to funding from the Ministry of Business, Innovation and Employment, researchers from five organisations — MetService, Niwa, Bodeker Scientific, Victoria University of Wellington and the University of Canterbury — have begun New Zealand’s own attribution project, called Extreme Weather Event Real-time Attribution Machine (EWERAM).
The team delved into the highly damaging Mid-Canterbury floods at the end of May and concluded the rainfall in that storm was 10% to 15% more intense as a consequence of human influence on the climate. They also found an event of this severity was at least 20% more likely to occur now than in pre-industrial times.
The floods of May 29 to 31 led to a state of emergency and were highly damaging across much of Canterbury, particularly between the Rakaia and Rangitata rivers, causing many tens of millions of dollars of damage. More than 200mm of rain fell in inland parts of the region, with a staggering 540mm recorded at Mt Somers in the headwaters of the Ashburton River.
MetService issued a rare red warning for the event, which was caused by a slow-moving low-pressure system feeding sub-tropical moisture on to the east of the South Island.
The findings are of immense value, although the researchers caution against using these Canterbury storm attribution figures for all rainstorms. The next event may not be affected by climate change to the same extent, given every bit of weather has its own characteristics. The team is now looking at the Westport floods in July.
For policymakers, planners, emergency managers and regional and local councils, knowing how much more severe the weather may be due to climate change will help with preparedness for the next, inevitable, storm.
Armed with such figures, planners will be able to recommend no-building zones in increasingly flood-prone areas where insurance will become hard to secure. Councils could also reconsider how high and extensive their flood-protection schemes might need to be.
It has been obvious for some years that human-induced climate change, and warming temperatures caused by greenhouse gas emissions, are affecting our weather.
That is especially the case when it comes to extreme weather events, such as torrential rain, heavy snow and heatwaves.
If ever there was doubt before, there can be none now.